1. FIELD OF THE INVENTION
The present invention relates to a loop antenna used for a portable, compact radio device.
2. DESCRIPTION OF THE PRIOR ART
Antennas excellent in portability and free from degradation of antenna gains even upon being carried by users have been used in conventional portable, compact radio devices. Among these antennas, a loop antenna disclosed in, e.g., Japanese Unexamined Utility Model Publication No. 5-2425 is most popular. For example, as shown in FIG. 1, this antenna is obtained by forming a band-like antenna conductor 1 into a loop. The loop antenna is disposed in a plastic housing 2 of a radio device such that the opening of the antenna conductor 1 faces outward. At the same time, the loop antenna is coupled to a parts board 4 of the radio device through a feeder 3. This antenna is excellent in portability because it can be incorporated in the housing 2. When the device is carried by a user, a higher antenna gain than that in a free space can be advantageously obtained due to a human body image effect. The human body image effect implies such phenomenon that an image loop is produced when the radio device is carried by a user, so that current in phase with an actual loop will flow and a peak of electromagnetic intensity will appear in the vicining of a human body by the image antenna in phase with the actual loop.
Since the loop antenna, however, is a magnetic field type antenna, the effective opening surface decreases upon receiving the influence of a metal object located close to the antenna opening. A radiation resistance decreases with respect to the loss resistance of the antenna. As result, in a portable, compact radio device in which an antenna is located very close to a radio unit, the influences of the device board and device parts undesirably lower the antenna gain.
To solve the above problem, the loop antenna must be located at a physically remote position from the radio unit. As the radio unit is, however, basically connected to the loop antenna through only the feeder, the strength of the antenna itself cannot be assured at the physically remote position from the radio unit.
To solve the above problem, it is desirable to arrange the antenna at the inner surface portion of the housing. The physical size of the antenna must be reduced due to the layout of the antenna at the inner surface portion of the housing, higher performance of a recent radio device, and an increase in packing density which result from downsizing. As a result, it becomes difficult to maintain conventional reception characteristics because the reception characteristics can be improved in proportion to an increase in effective size of the antenna. That is, in a conventional loop antenna, downsizing of the antenna is the direct cause for degrading the antenna characteristics.